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Table Of Contents  CertiGuide to Security+
 9  Chapter 1:  General Security Concepts (Domain 1.0; 30%)
      9  1.2  Authentication

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1.2.1  Kerberos
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1.2.3  Certificates
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1.2.2  Challenge Handshake Authentication Protocol (CHAP)

The Challenge Handshake Authentication Protocol, described by RFC 1994, authenticates a user by way of a “three way handshake”. First, the server sends the client a challenge message. (i.e., the server “challenges” the client.) Second, the client uses the message that is sent, along with the ID and the secret (the user’s password), to create a special code value called a hash, typically using MD5, and sends the hash back to the server. (i.e., the client responds with the other half of the “challenge handshake”.) Third, the server performs the same hashing function. In theory, the MD5 hash values will be equal, which gives authentication. This is repeated at random intervals during the session. By changing the ID value with each session, a replay attack is not possible. See 4.1.1 for explanations of hashing and MD5.

CHAP is most often used for PPP authentication. Firms such as Cisco and Microsoft have produced variations on the basic CHAP model, such as Microsoft’s MS-CHAP, with extensions specific to the Windows NT environment.

[spacer]CHAP Issues

Although like Kerberos, CHAP avoids sending the password over the wire, it still has security issues. In particular, the challenge/response mechanism is only as strong as the secret used to calculate the response. This means that users still need to choose good passwords – for example, not words that appear in any dictionary. Also, the protocol itself contains some weaknesses, including information disclosure, as documented in the paper, “Cheating CHAP.”
58


CHAP 3-Way Handshake

CHAP uses a 3-way handshake in which the server sends the client a “challenge” message, the client responds with a message encrypted into an MD5 hash using its password, and the server verifies that the message was encrypted with the right password by encrypting the same message with its version of the user’s password and making sure both messages’ hash codes match.

CHAP protects against session hijacking by performing this handshake at random times during a session.


Figure 5: CHAP can re-challenge at random times.

 


 __________________

58. Krahmer, Sebastian, “Cheating CHAP”, http://packetstormsecurity.nl/groups/teso/chap.pdf, February 2002.

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1.2.3  Certificates
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